Field of the Invention
The invention relates to a circuit for the glitch-free changeover of digital signals with a multiplexer that has a multiplicity of signal input terminals, signal select terminals, and a signal output terminal, and with a terminal for the circuit output.
In frequency synthesizers of microcontrollers having internal clock signal generation by means of PLL (phase-locked loop), there is often the need to enable an output clock signal of the circuit to be changed over between a plurality of frequencies. For this purpose, a multiplexer is usually used in the circuit. Different clock signals are thereby applied to different inputs of the multiplexer. At the multiplexer, it is possible to select which of the input signals is passed to the output. For this purpose, the multiplexer has corresponding signal select terminals. In many cases, it must be ensured that in the event of a changeover from a first input signal to a second input signal of the multiplexer--in the event of a changeover from a first clock rate to a second clock rate--no clock period occurs which is shorter than the minimum permissible clock period. Such an impermissible clock period is referred to in the art as a "glitch". Such a short pulse can lead to an uncontrolled malfunction of the downstream circuit which is driven by the output clock signal of the multiplexer. This is the case, in particular, when the clock signal is the system clock signal of a microcontroller.
The formation of glitches and their undesirable consequences are explained for example in the textbook by Paul Horowitz entitled "The Art of Electronics", 2nd Ed. 1989, Cambridge University Press, pp. 515ff. Horowitz describes (Ch. 8.19) synchronizers with which the problems caused by glitches can largely be avoided. The textbook by Carver A. Mead entitled "Introduction to VLSI Systems", Addison-Wesley, 1980, likewise discusses the subject of a lack of synchronization and its consequences in Chapter 7.5 thereof, entitled "Synchronisation Failure", pp. 236-242. The foregoing publications are incorporated by reference particularly with regard to the explanation of the term "glitches".